Added readme

README.md

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+# Reverse Polish notation calculator in .NET C#
+
+Pass RPN expression as one string as an argument.
+
+## Example
+
+```powershell
+dotnet run "5 3 +"
+# >> 8
+
+dotnet run "10 2 - 5 +"
+# >> 13
+```
+
+P.S.I still hate OOP, so code is rather imperative